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Why TRI-FIN?

A Tri-fin or Thruster set up is the most versatile and best performing fin set up for most types of sailing but top speed or freestyle. Because of our pretwisted side fins we can use bigger side fins without increasing drag. Like this we can make a board turnier and grippier than a quad with far less drag. Or looser than a twin. But we can also make it easier and more forgiving by using a bigger centre fin and smaller side fins. Having more fin area with less depth gives the best combination of upwind performance with control and maneouvrability.

Why Pre-Twisted Side fins?

In short: The board has an influence to the water flow underneath. The downwards pressure we put on the board cause the water to flow from the centre out to the rails. This effect is the biggest directly under the bottom and disappears going deeper. So a side fin needs an angle pointing inward towards the nose (toe-in angle) directly at the fin base and then slowly need to twist to be pointing straight at the tip. Like a sail needs to twist as there is less wind down below than in the top. A sail would perform horrible without twist. It is the same with fins. Just that it is not visible does not mean it is not there.
Everywhere you hear that tri-fins or quads are slow. That is not the case with the Witchcraft pre-twisted fin system. Often for normal sailors they are faster than a single fin due to the increased fin area with improved control.

Turning

Besides drag, also for the turning and wave riding behaviour is it very important to have the fins set up right. When the side fins do not have enough toe-in angle, without a side ways sail pressure, they deliver lift outward. A board turns because the tail is pushed out and the nose comes in. So with outward fin lift, it is the outer fin in a turn that helps the turn and the inner fin works against the turn. Then when the outer fin leaves the water in a turn, this results in the so called “straight lining effect” when the inner fin straightens out the turn.

NOW

Over the years we have kept on developing and refining the fins. Asymmetrical profiled side fins for increased drive and grip. The right amount of rake to improve twist, which adds drive, grip and predictability.  Now we are offering the latest version, developed for the use of colored G10 to have a strong, responsive, drivy fin with good flex but not too slippery to offer high precision so you can hit the lip where you are aiming for. Colored G10 is slightly softer than natural G10 and has the perfect flex.

We started working with trifins in 1997, during the first twinser hype. From there on we have been refining the set up with over 20 years of research development, using both trial and error and scientific methods: CFD (computer fluid dynamics),
underwater filming, the 4wfs.
No other brand has spend so much on research like we have, not even 30%.
Bad set up fin systems are recognisable by only offering small side fins. Even today, our fin system is still unique and unmatched.

CFD and underwaterfilming not only confirmed that pretwisted fins are needed but it also showed how much and what other influences there are to the water flow. It depends on the bottom shape, position of the fin and the average speed of a board. And also asymmetrical fins need to have the angles adapted. Nowadays CFD programs have become very accurate and offer various advantages over drag tank testing: It is possible to “see” the water flow directions and speeds everywhere and measure the performance of each fin separately rather than just being able to feel the fin cluster as a whole.

Apart from drag another important issue to have flow optimised side fins is turning. If the side fins are not set up with enough angle, they will be giving outward lift even without putting pressure on them. The ideal would be to set the side fins up so that when there is no sideways force applied from the sailor or sail, they simply cut through the water without providing any lift. This is how it should be, it gives the least drag and is the ideal starting situation to turn. In regular B&J sailing, when comparing to a single fin set up, the trifin proved even to be faster on any course, probably due to the increased control. As fins are small, this may seem insignificant but remember fins handle the same amount of power as a sail which is much bigger.

Then when a force is applied by the sailor or sail, all fins will do a similar amount of work:

The small differences stem from the position and asymmetry of each fin.

Paralel placed symmetrical side fins do give lift even when no pressure is applied by the sailor:

Then, when in a turn the outer fin ventilates and thus losing lift, the inner fin will be steering against the turn, making the board want to straight line.

Computer generated images of a bottom turn to show how realistic CFD technology can simulate situations like this nowadays. CFD also gives images of flow directions, flow speeds, pressure distribution over the board and fins, ventilation, tip vortexes, water and air mixtures and points of increased turbulence or drag but we are not going to give it all away…..

Since fin lift increases exponentially with the speed, you will notice this a lot more the faster you go and less at lower speeds.

So the ideal starting situation for entering a turn is that none of the fins are giving lift when there is no pressure applied to them, like a centre fin already does naturally. From this as soon as any pressure is applied to the fins, each will do an equal amount of “work”.

All profiles are developed to handle the widest range of Angle of attack as possible. Essential for wave sailing plus you are able to choose smaller fins and they are easier to get planing.

The side fins have a wing like cambered asymmetrical profile so the inner fin can handle higher pressures at lower drags in a turn. Very important for using asymmetrical foils is to adapt the angle of attack since an asymmetrical foil will provide lift already with zero angle of attack, like airplane wings do as well. So you need to readjust the angle to where the fin has the least drag and does not provide lift again. The amount of adjustment depends on the amount of asymmetry. Without the use of CFD this is very hard to find by trial and error since you can only feel what the whole fin cluster is doing, not each fin separately.

The above picture also illustrates why a tri fin set up provides more predictability/control. If one of the side fins is ventilated, you only lose 25 to 33% of fin area holding the board in the turn. Would you want the tail to break out sooner, you could put a smaller centre fin and bigger side fins.

All fins are swept back so they adapt better to the situation, handle tight turns better and are generally easier to sail. Compare the amount of fin flex of a carbon fin in a soft top turn:

Camera set up for underwater filming of the side fin

Top turn image, note the angle of the horizon and the difference in fin flex.

Bottom turn image with fin ventilation due to the top of the fin having left the water.